John Birt's memoirs present his best opportunity to set the record straight about his controversial BBC career, and to answer his many critics, myself included. What emerges, after 500-odd pages, is a rather disturbing portrait of a man who loses his soul when he is appointed director general of the BBC and assumes a key role in public life at the heart of the British establishment.

I am with him and for him all the way through his remarkable upward journey from humble beginnings in Liverpool, through a brutal education at the hands of the Christian Brothers, on to Oxford University and then a dazzling television career at Granada and London Weekend, where I first worked with him.

But as soon as he arrives at the BBC, something inside him changes. Instead of the gifted programme visionary, he begins to see himself as the messiah called to cleanse and save the decadent, decaying and doomed corporation; to instill discipline and rigour, and bend it to his will: the only will.

The arrogance by this point is suffocating and, worse, his contempt for the people who created the legacy he inherited downright shameful. The key to his mindset is to be found in the title of this book, taken from a Himalayan saying: "When the road through the mountains forks, take the harder path."

Er, why? Not a question that occurs to him at the helm of the BBC. He seems to positively thrive on fierce criticism, both personal and professional. Counter views serve only to make his path harder, and therefore, in his mind, more right. It is an old trick: to exaggerate a problem then promote yourself as being the only person capable of solving it. Birt proved it is still effective, especially when your masters are politicians.

Westminster has always been biased against understanding the BBC, and open to simplistic and prejudiced notions about its purpose and function. "The BBC is out of control" is a phrase that falls easily from the mouths of politicians who refuse to understand why, as its ultimate paymasters, they can't control it.

"Saviour" Birt played to these prejudices: he told them exactly what they wanted to hear. The place is a shambles, the old order has let it go, full of waste... fill in the rest yourselves. Out front, his parliamentary gallery loved it, and loved him.

Yes, the BBC of the 1980s needed a reforming hand to lead it into the new world of greater competition, the Thatcher and post-Thatcher era. But the first great Birtian myth is that he and he alone had the vision and strength of character to effect this modernisation. It is just not true. Most inside the organisation were ready to make the necessary changes in efficiency and structure. Surely it would not have been difficult to convince producers that better organisation would free up more cash for programmes?

The easier path was to manage change by informed consent; we were ready for change, believe me. I was there. Instead, Birt chose the harder path, thus more easily to reassure his political masters that he was, indeed, their new messiah. More mullah, actually, as it turned out.

He set out to impose a set of reforms that made little sense to those of us who had left, and even less sense to the majority who stayed. All dissent was squashed. If you were inside and complained, you were branded a traitor, and banished to the gulag. If you were outside and questioned him, you were dismissed as being anti-change, one of the old guard responsible for the mess he was clearing up.

Writing of my public attempt, at the Edinburgh television festival in 1992, to question his zealous approach to reform Birt now writes: "History would prove Grade wrong in every respect, but in 1992 his hysteria played well in the hall..." Well, not quite. Producer choice, bi-media journalism, micro-internal markets - all the paraphernalia of "Birty's bollix" has come apart and been hastily dismantled since his departure.

The second myth he perpetuates in this book is that he saved the BBC from the wrath of the Tories during Thatcher's reign. I don't think so. Yes, the Iron Lady loved rattling the corporation's cage. But the moment of gravest political danger was the Peacock report, set up by Mrs Thatcher and designed to deliver a BBC at least partly funded by advertising - a first step to privatisation.

The BBC saw off the Peacock reforms after the most intense battle for the survival of the licence fee it had ever had to face, against a background of unprecedented political and ideological hostility. It was a magnificent "win" - and it all happened two years before our self-invented messiah arrived at the BBC.

The third Birtian myth is that he "cleansed" BBC journalism. It may not have been perfect before he arrived, but at least it was independent and editorially audacious. His control over the journalism of the BBC was more interventionist and heavy handed than at any time in its history. Peter Jay's Panorama, just before the 1992 election, was pulled on Birt's orders - perhaps because it was too critical of Tory economic policy? A bit like dropping a programme called "The Road to Wembley" the week before the cup final.

To support him in his crusade, Birt recruited a team of talented outsiders to complement the young BBC "lifers" he had marked as followers and promoted. Ian Hargreaves came from the Financial Times, Liz Forgan from Channel 4 and Bob Phillis from ITN. None of them stayed. I wonder why - but, curiously, Birt doesn't.

Two conclusions about Birt's time at the BBC emerge vividly from his account. The first is his sense of certainty. Now, we all think we are right from time to time. But do we ever really know we are right? When his predecessor as DG, Mike Checkland, queries some recommendations in a Birt report, he writes: "I had to draft a minority report. I was furious. I had worked on the study for the best part of nine months. The rigour of the analysis was flawless. The case was compelling." Echoes of Captain Queeg (Humphrey Bogart) unravelling at the court martial in The Caine Mutiny: "I worked it out with geometric logic, someone had been stealing the strawberries... " Cue the steel balls! A fine line divides certainty and zealotry. There is no evidence in the book that our author understands this.

The second conclusion is that Melvyn Bragg, a mutual friend and former colleague of ours, was right when he said that Birt can walk into a room and find the power like a heat-seeking missile. Throughout his book, Birt reinforces Melvyn's acute observation: "I went to see Mrs Thatcher shortly after her resignation in her splendid new home in Chester Square to ask her to record her memoirs for the BBC... She was full of energy and spirit, and I was among the first to hear - it had not yet been reported - that she was already on John Major's case, disapproving of the actions of the new prime minister on a wide front... Her candour made John Whittingdale, her political secretary, squirm. She waved him away impatiently: 'Don't worry. John's trusted!' " Is that the way a DG of the BBC should be happy to portray himself?

The book is littered with references to his intimate (not just professional) relationships with powerful folk. Cabinet secretaries, ministers of state, Tony this and Cherie that... my dear, the name-dropping! On the plus side, he can look back with immense pride at his embracing of the digital future: laying down the foundations for www.bbc.co.uk, and for a successful licence fee negotiation to fund it.

Is the clue to John Birt's transformation when he arrived at the BBC simply that old corrupter, power? He gives the impression in all the BBC chapters of a man who enjoys power, seeks power and relishes mixing with power. The BBC was simply his vehicle.

Perhaps the Christian Brothers have something to answer for. Or maybe it's a gene. I am neither a scientist nor a psychologist. All I know is that the John Birt I worked with for eight fulfilling years at London Weekend Television is a man for whom I still carry the deepest affection, respect and loyalty. The Birt I left at the BBC is someone I didn't recognise, didn't like and could not respect.

Nothing in this very readable and important book changes my feelings. As far as I am concerned, he became a lost soul, and a great loss to British broadcasting. But do read it, I might be wrong.

· Sir Michael Grade is chairman of Camelot and Pinewood and Shepperton Studios.